Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a

•Earth system model constraints on OAE 1a carbon emissions sources and fluxes.•Modest organic carbon-like emissions initiated the negative carbon isotope excursion.•Thereafter massive emissions from a mantle-like source responsible for pCO2 increase.•New Os record & modelling confirm OAE 1a onset precedes onset of large-scale volcanism.•First Mesozoic emissions constraint independent of carbon source/climate sensitivity.